Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has.
If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.
I think; therefore I am.
I doubt, therefore I think, therefore I am
Doubt is the origin of wisdom.
And thus, the actions of life often not allowing any delay, it is a truth very certain that, when it is not in our power to determine the most true opinions we ought to follow the most probable.
― Discourse on Method
Some years ago I was struck by the large number of falsehoods that I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.
― Meditations on First Philosophy
The greatest minds are capable of the greatest vices as well as of the greatest virtues.
Whatever I have up till now accepted as most true and assured I have gotten either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once.
― Meditations on First Philosophy
For these reasons, as soon as my age permitted me to pass from under the control of my instructors, I entirely abandoned the study of letters, and resolved no longer to seek any other science than the knowledge of myself, or of the great book of the world. I spent the remainder of my youth in traveling, in visiting courts and armies, in holding intercourse with men of different dispositions and ranks, in collecting varied experience, in proving myself in the different situations into which fortune threw me,
― The Works of Rene Descartes
My third maxim was to endeavour always to conquer myself rather than fortune, and change my desires rather than the order of the world, and in general, accustom myself to the persuasion that, except our own thoughts, there is nothing absolutely in our power;
― The Rene Descartes Collection: His Classic Works
In order to determine whether we can know anything with certainty, we first have to doubt everything we know.